The present invention relates to a soot blower for cleaning the surfaces of a heat exchanger.
In such soot blowers, when not actuated, the corrosive hot gases carrying soot from the combustion of solid, liquid or gaseous fuels come into contact with the soot blower lance, and through the blower jet openings in the lance can enter the lance tube, where they cause damage by corrosion or fouling.
To reduce this problem, an inert gaseous purging medium, for example, air, is introduced beyond the soot blower valve when the blower is inoperative, with the pressure of the purging medium being greater than that of the combustion gases in the heat exchanger. A stream of the inert purging medium thus passes through the lance into the heat exchanger, and prevents the entry of the corrosive gases at the jets. A sealing medium, for example, air, is also blown into the wall casing surrounding the entry point of the lance into the heat exchanger to provide a seal against escape of combustion gas to the environment.
In heat exchangers, particularly those for power station boilers, which are provided with several soot blowers, both the purging air and the sealing air are drawn from a central blower system and distributed to the individual soot blowers. This requires an expensive ducting system. The relatively low pressure level of the central blower frequently makes it necessary to use large-diameter ducts to reduce frictional losses. A special control element is needed ahead of each soot blower to achieve even distribution of the required amounts of air, a difficulty which is compounded by the different frictional losses resulting from the unequal duct lengths to the individual soot blowers. Finally, the connections at the soot blower valve and/or at the wall casing must be flexible to allow for movement of the soot blower and the effects of expansion.
A further disadvantage of such central air systems, is that very often the purging and sealing air is drawn from a blower which is normally shut down when the heat exchanger is switched off so that the purging and sealing is then no longer present. This means that the stove effect which persists within the hot heat exchanger drives the undesirable combustion gases into the soot blower lance or from the wall opening of the heat exchanger into the environment.
It is a further requirement of many manufacturers and operators of heat exchangers that the quantity of air introduced at each soot blower should be as low as possible and should not exceed a quite low maximum value even in the event of those pressure variations which typically frequently occur on the combustion gas side.